When a disaster such as power failure, fire, or the like, by which a data center is affected in its entirety, occurs, a disaster recovery technology is used to enable a user who executes business through a server connected to him from a remote place, to continuously execute the business. A conventional disaster recovery technology (technology for avoiding failure caused by disaster) includes a method of switching an operating server, which provides service, to a remote server by using technology such as a dual operation, a process migration, or the like.
As shown in a patent document 1, the process migration is a technology for migrating a program operating on a computer to another computer while maintaining the executing status of the program and causing the program to be continuously executed in the same status as that in the original computer.
Further, as shown in a patent document 2, there are also known a VLAN switching technology and the like in which a gateway of a user site switches the connection destination VLAN of a user, as a technology for switching the connection destination target network of the user that has accessed to a main server to a backup server and switching an operating server while maintaining a connection between the user and the server or between the user and a process.
Patent documents 3, 5 and 6 disclose technologies for migrating an IP address when a current system is switched to a waiting system in a dual operation arrangement. Further, a patent document 4 discloses a technology in which a client executes communication by converting a connection destination IP address from a virtual IP address to an actual IP address, and, when failure occurs, the actual IP address to be converted is switched from an actual IP address of a current (main) system to an actual IP address of a waiting (backup) system.    Patent document 1: JP-A-08-329025    Patent document 2: JP-2003-375352 (JP-A-2005-142702)    Patent document 3: JP-A-08-335198    Patent document 4: JP-A-09-034814    Patent document 5: JP-A-09-091233    Patent document 6: JP-A-10-320323